This invention relates to a brake shoe anchor device applicable to an internal drum brake device, particularly to a leading-trailing (LT) type, a two-leading (2L) type, or a single shoe type internal drum brake device.
A conventional LT type drum brake device as shown in FIGS. 8 and 9 has a pair of brake shoes 20, 30 mounted on a back plate 10 and elastically retained by shoe holding mechanisms 11, 11.
In FIG. 8, the upper adjacent ends of the brake shoes 20, 30 respectively abut against a hydraulic cylinder 40 while the lower adjacent ends of the brake shoes 20, 30 abut against an anchor device 13, Further, a shoe return spring 12a extends between the upper inner sides of the brake shoes 20, 30 while a shoe return spring 12b extends between the lower inner sides of the brake shoes 20, 30.
Upon pressurizing the hydraulic cylinder 40 to push the upper adjacent ends of the brake shoes 20, 30 outwardly, the brake shoes 20, 30 spread open with points of abutment P, P on the anchor device 13 as the fulcrum. Then, linings 23, 33 of the brake shoes 20, 30 frictionally contact with an inner circumferential surface of a brake drum (not shown in the figures), thereby generating a braking force.
The braking force generated by the conventional LT type drum brake device depends on a brake drum inner radius, on an activating force applied to the brake shoes by the pressurized hydraulic cylinder 40, and on a brake factor. In case of the drum brake device shown in FIG. 8, the brake factor is known as a non-dimensional number which varies in accordance with a distance H1 from a horizontal line passing through a brake center O between the brake shoes 20, 30 to the supporting point P of the respective brake shoes 20, 30 on the anchor device 13 as shown in FIG. 8.
Adjacent to the hydraulic cylinder 40, a strut 50 with an automatic shoe clearance adjustment mechanism extends between the brake shoes 20 and 30. The left end of the strut 50 engages a shoe web 21 of the brake shoe 20, and the right end of the strut 50 engages a shoe web 31 of the brake shoe 30.
The structure of the strut 50 is briefly explained with reference to FIG. 9. The strut 50 is a one-shot automatic shoe clearance adjustment device which comprises an adjustment plate 51, a bell crank lever 52, a pin 55 and two springs 53, 54. The adjustment plate 51 has a notched groove 51a at the left side, and an inner edge of the shoe web 21 abuts against the bottom of the notched groove 51a. 
Small teeth 51b are formed on an intermediate portion of the adjustment plate 51. The bell crank lever 52 is rotatably pivotally supported relative to the adjustment plate 51 by the pin 55 in the intermediate portion thereof and longitudinally movable along the surface of the adjustment plate 51. Small teeth 52b formed on an outer edge of a fan-shaped arm 52a of the bell crank lever 52 engage the small teeth 51b formed on the adjustment plate 51.
In addition, another arm 52c of the bell crank lever 52 with a cam surface is freely fit into a rectangular hole 32 formed on the shoe web 31 with a gap at the left side of the rectangular hole 32.
A first spring 53 extends between the shoe web 21 and the adjustment plate 51, and a second spring 54 extends between the adjustment plate 51 and the pin 55. The first and the second springs 53, 54 are so designed that a mounting load of the first spring 53 is higher than that of the second spring 54.
During braking, the adjustment plate 51 and the bell crank lever 52 move to the left together with the brake shoe 20 due to the spring force of the springs 53, 54. At this time, as is well-known in the field, if an amount of the wear of the linings 23, 33 exceeds an amount of the gap between the other arm 52c and the rectangular hole 32 plus a height of the engaged small teeth 51b, 52b, the bell crank lever 52 rotates counterclockwise, as shown in FIG. 9, for one pitch of the teeth to automatically adjust the clearance between the brake drum and the linings 23, 33, thereby maintaining the clearance to be always constant.
The above-described conventional drum brake device has the following points to be improved:
 less than A greater than  During braking, a braking force actually acts at the time when the brake shoes 20, 30 frictionally contact with the inner circumferential surface of the brake drum. The time from the beginning of the brake shoe opening to the contact of the linings thereof with the brake drum is said to be the xe2x80x9cdelayxe2x80x9d.
Furthermore, since the braking force increases in relation to the pressure supplied to the hydraulic cylinder 40, in case of a gentle incline of the braking force increase, the response time before the driver actually feels a braking action or deceleration is even longer, thus giving the vehicle driver an impression of an even longer delay in braking effectiveness and thus an insecure feeling.
 less than B greater than  Depending upon the materials constituting the linings 23, 33, wetness on the frictional surfaces of the linings 23, 33 and the brake drum causes an overbraking effect compared to the braking effect under a normal condition. This may also make a vehicle stop with a jerk or a series of jerks, which may lead to a dangerous situation by making a driver to hit his/her face on the steering wheel or by moving freight or loose items inside the vehicle.
The object of this invention is to provide a brake shoe anchor device for an internal drum brake device which resolves the problems mentioned above. More specifically, this invention aims at providing a brake shoe anchor device suitable to be employed in a drum brake device which eliminates the problem of the delay in the braking effectiveness or over-braking and which gives the driver a more secure feeling.
According to the invention, there is provided a brake shoe anchor device for an internal drum brake device as defined in claim 1. Preferred embodiments of the invention are defined in the sub-claims.
A first aspect of this invention is to provide a brake shoe anchor device for supporting an end of a brake shoe in an internal drum brake device, comprising two anchor members located at radial different positions with respect to a center of the brake device, one is a rigid anchor member having a rigid anchor surface for rigidly supporting the end of the brake shoe and the other is a deformable/resilient anchor member having a deformable anchor surface for resiliently supporting the end of the brake shoe, wherein the two anchor members are arranged such that the brake shoe is supported by the deformable anchor member before a braking force reaches a predetermined value, and such that the brake shoe is supported by the rigid anchor member when the braking force exceeds the predetermined value.
A second aspect of this invention is to provide a brake shoe anchor device as above wherein the deformable anchor member is positioned radially closer to a brake center than the rigid anchor member.
A third aspect of this invention is to provide a brake shoe anchor device as above wherein the deformable anchor member is positioned radially farther from a brake center than the rigid anchor member.
A fourth aspect of this invention is to provide a brake shoe anchor device as above wherein the deformable anchor member is comprised of an elastic material.
A fifth aspect of this invention is to provide a brake shoe anchor device as above wherein said deformable anchor member comprises a coil spring.